It is becoming widely accepted that radar polarimetry provides accurate and informative weather measurements, while phased array radar (PAR) technology can shorten data update time. This suggests that the future weather radar should have the functions of both polarimetry and electronic steering capabilities, i.e., Polarimetric Phased Array Radar (PPAR), allowing multi-missions of weather surveillance and target detection.
In addition to military applications for target recognition and tracking (Brookner 2007), Phased Array Radar (PAR) technology has recently been successfully introduced to the weather community. A phased array weather radar, the National Weather Radar Testbed (NWRT) operating at a wavelength of 9.38 cm, was developed in Norman, Okla. through a joint effort of a government/university/industry team (Zrnic et al. 2007). The NWRT demonstrated that its pulse-to-pulse electronic beam steering capability enables as accurate meteorological measurements in shorter storm surveillance times as achieved with a conventional dish antenna having a mechanically steered beam. The shorter surveillance times result in faster data updates and the capability to observe detailed evolutions of severe storm phenomena (Yu et al. 2007; Heinselman et al. 2008). The NWRT also has a hybrid capability to both mechanically and electronically steer the beam. This capability has allowed multi-pattern measurements of the same meteorological volume to successfully mitigate both stationary and moving clutter (Zhang et al. 2010). Furthermore, the NWRT uses an antenna from the AN/SPY1-A monopulse radar of the Aegis system (Sherman, 1988), which has sum and difference channels; these can be combined to implement Spaced Antenna Interferometry (SAI) techniques for crossbeam wind measurement (Zhang and Doviak 2007), and subvolume inhomogeneity/object detection (Zhang and Doviak 2008). It has been also theorized that the AN/SPY1-A auxiliary channels could support implementation of adaptive clutter cancellation techniques (Le et al. 2009).
While PAR technology has recently received wide-spread attention in the weather community, weather radar polarimetry has matured to a point that it is being implemented on the national network of WSR-88D Doppler radars (Doviak et al. 2000) using its conventional dish antenna. Polarimetric radar provides multi-parameter measurements that reveal detailed microphysics of storms in addition to hydrometeor classification, accurate precipitation estimation and improved weather nowcasts. Therefore, the weather community and the nation expect that the future Multi-function Phased Array Radar (MPAR) will retain all the capabilities of the polarimetric WSR-88D (Smith et al. 2008). It is the polarimetric capability which the 2nd MPAR symposium (http://www.ofcm.noaa.govimpar-symposium, 17-19 Nov., 2009, Norman, Okla.) identified as the most challenging technical issue that the community is facing. The challenge comes from the fact that highly accurate polarimetric radar measurements are required to provide meaningful information. But biases inherent to Planar Polarimetric Phased Array Radar (PPPAR) exist and can be larger than the intrinsic values if the beam is directed away from the planar array's broadside. For example, the intrinsic ZDR values range only from about 0.1 dB for drizzle and dry snow to 3-4 dB for heavy rain and large drops. Thus, it is desirable that the measurement error for ZDR be of the order of 0.1 dB (Zhang et al. 2001, Brandes et al. 2003). But the ZDR bias for a PPPAR can be a few dBs (Zhang et al. 2009a). Hence, it is crucial for the success of the MPAR project that the system configuration for a PPPAR is selected correctly and designed optimally.
In the presentation at the 34th AMS radar conference, a number of issues with PPAR for weather measurements have been listed and discussed, including sensitivity, bias, calibration, cross-polar isolation, array configuration, polarization mode selection, waveform optimization, and signal processing and display. The polarization bias was quantified and a calibration procedure was proposed by Zhang et al. (2009) for planar arrays. The other issues remain.
As background, a variety of antenna array configurations exist including linear array, planar array, circular/cylindrical array, and spherical array. The linear array needs one mechanical rotation for weather surveillance like the rapid Doppler On Wheels (rapid DOW) (Wurman 2003) and the proposed design for CASA (Hopf et al. 2009). For the planar array, multiple faces (normally four) are needed (e.g., the SPY-1A). But the planar array has sensitivity loss and polarization bias if the beam points away from the broadside (Zhang et al. 2009a). Antennas having circular or cylindrical configurations have been used for direction finding and communications (Royer 1966; Raffaelli and Johansson 2003) but not with a dual-polarization phased-array radar, (multi-function or single function), for weather related tasks. For satellite communication applications, the spherical array is optimal and flexible in its use of the antenna aperture size and in its symmetry (Tomasic et al. 2002).
With respect to PPAR, possible antenna array configurations for PPAR include linear array, 2D planar array, and spherical array. The linear array needs one mechanical rotation for weather surveillance like rapid DOW (Wurman, 2003) and the proposed design for CASA (Hopf et al. 2009). For planar array, multiple faces (normally four) are needed (e.g., the SPY-1A or, as with the NWRT a composite of mechanical and electronic beam steering. The planar array has issues of sensitivity loss and polarization bias when the beam points away from the broadside (Zhang et al. 2009). A spherical array is optimal and flexible in terms of using the antenna aperture and has the symmetry in all the directions (Tomasic et al. 2002) needed for receiving signals from satellites. For weather applications, however, the spherical array can have problems in making polarimetric measurement of weather because high cross-polar isolation is required.